It is known from U.S. Pat. No. 4,701,237 to confer a perforated structure on a pre-fabricated nonwoven with the unperforated nonwoven being slit several times lengthwise, then stretched laterally to produce the holes, and fixed in this form by heat. A perforated nonwoven of this kind has sharply delimited holes but little lateral strength.
The type of manufacture according to U.S. Pat. No. 3,750,237 is better in this regard, with the prefabricated nonwoven being held between two endless webs and impacted by strong water jets to produce the perforated structure. The desired perforated structure is mounted on the endless web, such as a drum. It consists of a uniformly perforated drum covered all the way around by an endless screen. The endless screen has open and closed areas, depending on the desired hole structure. The disadvantage of this method of producing the holes lies in the fact that no holes with sharply delimited edges can be produced in this fashion and individual nonwoven fibers are pushed toward the endless screen by the strong water jets when the holes are produced and then are caught between the wires of the screen. In addition, the strength of the perforated nonwoven is low since the width of the ribs between the holes is nonuniform. A closer examination of the nonwoven structure reveals disadvantageous interference bands. These are produced by the holes in the drum that have the screen stretched over them and the fixed arrangement of the jets in nozzle beam A.
Sharply delimited holes can be produced subsequently in a prefabricated uniform nonwoven only with the type of manufacture according to EP-A-0 215 648, 0 223 614, or 0 273 454. In each case, a perforated drum is made from a smooth piece of sheet metal with drainage openings, on which drum plastic elevations are provided between the openings and are distributed uniformly over the surface. The plastic elevations can consist of beads that are open halfway, so that drainage openings are also formed at the same time, or of mandrels that taper to a tip at the top and are uniformly distributed, between which the drainage openings in the sheet are arranged in the form of holes. Although this drum can be used to produce the desired holes that are sharply delimited from the nonwoven, the strength of a nonwoven products punched to form circles is not uniform in all directions. The prescribed hole structure or the specified arrangement of plastic elevations in the form of a rhombus or the like prevents this dimensional stability. In addition, the feared interference bands can also appear on the treated nonwoven because the arrangement of the holes in the sheet metal of the drum can interfere with the necessarily permanent arrangement of the nozzle holes in the nozzle beam to form bands.